Hydraulic systems are frequently used for powering construction machines, such an excavator, which has a boom assembly comprising a boom, an arm and a bucket pivotally coupled to each other. A hydraulic cylinder assembly is used control and operate the boom assembly, wherein the hydraulic cylinder assembly comprises a plurality of hydraulic cylinders, each having a piston therein which defines two chambers in the cylinder.
During powered extension and retraction of a hydraulic cylinder, pressurized fluid from a pump is usually applied by a valve assembly to one cylinder chamber and all the fluid exhausting from the other cylinder chamber flows through the valve assembly into a return conduit that leads to the system tank. Under some conditions, an external load or other force acting on the machine enables extension or retraction of the cylinder assembly without significant fluid pressure from the pump. This is often referred to as an overrunning load. In an excavator for example, when the bucket is filled with heavy material, the boom can be lowered by the force of gravity alone. To save energy, it is desirable to recover the energy of that exhausting fluid, instead of dissipating it in the valve assembly. Some prior hydraulic systems operate in several different operating modes, of which one for example is said powered extension and retraction, and another is an energy recovery mode, in which pressurised exhausting fluid from an hydraulic actuator is sent to an accumulator, where it is stored under pressure for later use in powering the machine. Prior art documents US 2008/0110165 and US 2007/0074509 shows examples of energy recovery systems using such accumulators. These prior art systems are however not optimized and further improvements with respect to energy saving are possible.
There is thus a need for an improved energy saving system for recovering and reusing energy in a hydraulic system.